AIE 33 



human ingenuity has been devised to interfere with the 

 action of the wind and the penetration of sunHght. 



Under the influence of respiration the oxygen of in- 

 habited buildings is less than that of the outside air, for the 

 reason that it is being withdrawn, and unless proper 

 ventilation is establisTied it is not being replaced in suffi- 

 cient quantity to keep the air at a normal standard. A 

 closed stable has been known to give as low a percentage of 

 oxygen as 20'39. It may be said that very bad air begins 

 when the oxygen falls as low as 20 '6 per cent. 



Ozone which is only a highly active condition of oxygen 

 has been described as occurring in traces in the atmosphere, 

 the result of electrical conditions. 



Very little is known about it, even the tests usually 

 employed to detect its presence are now of doubtfiil value. 



Nitrogen constitutes about four-fifths of the atmosphere. 

 It is a gas remarkable for its purely negative properties ; in 

 our present state of knowledge it is regarded as a diluent 

 of the oxygen, but very little is known about it. It is quite 

 possible that what in the past has been regarded as nitrogen 

 may turn out to be a mixture of gases. Already one of 

 these has been discovered, viz., argon, associated to the 

 extent of one per cent, with nitrogen. 



Argon is as obscure as nitrogen, and just as inert ; no 

 element has yet been found which is capable of entering 

 into combination with it. 



It is very probable the nitrogen in the air has some 

 important function to perform in connection with plant 



life. 



Carbonic Acid in the air is mainly derived from the earth ; 

 the ground air contains 250 times more carbonic acid than 

 the atmosphere. The average amount of carbonic acid, 

 ■04 per cent., is not found to vary to any considerable 

 extent, though as might be expected samples collected close 

 to the ground contain more carbonic acid than air from the 

 tops of mountains, while the carbonic acid in the air at sea 



is still less. ■ t u ■ 



Though large variations in the carbonic acid of the air 



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